C-OTX2 IS EXPRESSED IN 2 DIFFERENT PHASES OF GASTRULATION AND IS SENSITIVE TO RETINOIC ACID TREATMENT IN CHICK-EMBRYO

We cloned the chick homologue of the mouse Otx2 gene, c-otx2, and anal yzed its expression pattern during gastrulation. During mouse embryoge nesis, Otx2 expression is first detected in the entire epiblast and af ter the formation of the primitive streak becomes confined to the most anterior region of the embryo corresponding to presumptive fore- and mid-brain. Similarly, two distinct phases of c-otx2 expression were ob served in the chick, c-otx2 transcripts were first detected in the uni ncubated egg and up to stage XIII, in all epiblast, and forming hypobl ast and mesoblast cells. During primitive streak progression, c-otx2 e xpression becomes progressively restricted to anterior regions and is mainly associated with Hensen's node. When the extension of the streak is maximal, transcripts are only found in Hensen's node. A second pha se of c-otx2 expression starts during streak regression. c-otx2 transc ripts are lost from the node and present in higher abundance in anteri or neuroectoderm and mesendoderm, with the exception of forming notoch ord and floor plate. The first phase of expression bears strong simila rity with that of c-gsc, a gene shown to be a marker for cells that ha ve organizer activity in the chick. Therefore, we compared the express ion of the two genes by double staining on the same embryo. This analy sis demonstrated that c-otx2 is transcribed first and its expression i n the hypoblast precedes that of c-gsc. On the other hand, c-gsc is an earlier marker of primitive streak cells. The expression domains of t he two genes transiently overlap in Hensen's node and anterior mesendo derm, whereas only c-otx2 is expressed in neuroectodermal areas. The s econd phase of c-otx2 expression is sensitive to an early treatment wi th retinoic acid. This treatment abolishes c-otx2 expression in mesend oderm and restricts it to most anterior regions in the forming neural plate. In conclusion, our results suggest that c-otx2 expression is fi rst associated with cells with an anterior mesendoderm fate and subseq uently extends to anterior neuroectoderm.